(1) Field of the Invention
The invention relates to an analog-to-digital converter (A/D converter) for converting an analog signal into a digital signal consisting of a sequence of code words each comprising a plurality of bits.
(2) Description of the Prior Art
A/D converters are used in widely divergent technical fields. Embodiments of A/D converters are described in reference 1, (see paragraph D). In general, they convert a time-continuous signal into a time and amplitude-discrete signal. For this conversion an analog signal is sampled with a certain sampling frequency. Each of the samples thus obtained is encoded within a certain period of time, the so-called conversion time. Encoding of a signal sample means that a code word or number is generated comprising a number of symbols or bits. In order that the required number of bits does not exceed a predetermined number, the sample is first quantized, that is to say that the value of this sample is made equal to an integral number of times a pre-determined elementary voltage or current. This elementary voltage or current is denoted quantization step size. The code word now indicates how many times the quantization step size is included in the quantized sample. The number of bits of each of these code words is determined by the distance between the highest positive and the lowest negative value of the analog signal to be coded, and by the desired quantization step size. In its turn this quantization step size determines the quantizing noise introduced by the quantizing process. As known, this quantizing noise is directly proportional to the quantization step size and will have to be below a predetermined threshold. The height of this threshold is determined when the type of signal to be coded is known. For example, when coding speech signals, a greater amount of quantizing noise will be tolerated than when coding music signals which must be of Hi-Fi quality after decoding.
In practice, the quantizing noise is not used as an absolute quantity but it is considered relatively with respect to the signal, more particularly the ratio between the signal and the quantizing noise is considered in practice and this ratio is expressed in decibels (dB). Hereinafter this ratio will be denoted by SNR and, as known, is approximately equal to EQU SNR=(6n-2)dB (1)
for a sinusoidal signal. Herein n represents the number of bits of each of the code words.
It follows from (1) that when a higher value of SNR is desired the number of bits n of each of the code words must be increased. As more quantizing noise can be allowed for coding a speech signal than for coding a music signal, twelve-bit code words are usually taken in practice for coding a speech signal but for the coding of a music signal at least 14-bit code words are required.
The more bits a code word must have, the greater the accuracy and stability must be of the components with which the A/D converter is implemented and, consequently, the higher its price. This price is a barrier for the use of these A/D converters in, for example, audio equipment such as, for example, magnetophons for the consumer market.
Hereinafter a code word comprising, for example, d bits will be denoted "d-bit code word". Similarly, an A/D converter arranged for converting an analog signal into d-bit code words will be denoted "d-bit A/D converter".